Field of the Invention
This invention relates to seismology and more particularly relates to the use of a single marine vessel for the monitoring of an ocean bottom seismic node, including the monitoring, positioning, and/or guiding of deployed autonomous seismic nodes.
Description of the Related Art
Marine seismic data acquisition and processing generates a profile (image) of a geophysical structure under the seafloor. Reflection seismology is a method of geophysical exploration to determine the properties of the Earth's subsurface, which is especially helpful in determining an accurate location of oil and gas reservoirs or any targeted features. Marine reflection seismology is based on using a controlled source of energy (typically acoustic energy) that sends the energy through subsurface geologic formations. The transmitted acoustic energy propagates downwardly through the subsurface as acoustic waves, also referred to as seismic waves or signals. By measuring the time it takes for the reflections or refractions to come back to seismic receivers (also known as seismic data recorders or nodes), it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits or other geological structures of interest.
There are many methods to record the reflections from a seismic wave off the geological structures present in the surface beneath the seafloor. In one method, a marine vessel tows an array of seismic data recorders provided on streamers. In another method, a series of interconnected sensors are integrated with a cable that is towed behind a marine vessel and placed onto the seabed, which is commonly known as ocean bottom cables. In another method, autonomous seismic data recorders are placed directly on the ocean bottom by a variety of mechanisms, including by the use of one or more of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), by dropping or diving from a surface or subsurface vessel, or by attaching autonomous nodes to a cable that is deployed behind a marine vessel. The data recorders may be discrete, autonomous units, with no direct connection to other nodes or to the marine vessel, where data is stored and recorded.
When an ocean bottom cable or autonomous nodes attached to a cable are deployed in the sea, it is desirable to know the position of the cable and the positions of the nodes during and after deployment. One common way to identify some portions of the deployed cable is to use acoustic positioning transponders that are selectively placed at various portions of the cable. In general, a transponder is a remote acoustic beacon that requires the use of expensive Ultra Short Baseline (USBL) technology, which calculates the range and bearing for each transponder. Subsea transponders may transmit an acoustic signal to a marine vessel that calculates the position of the transponders (thus indicating the position of the cable) on the sea floor using USBL (or similar) technology. However, many problems exist with the use of transponders. Because transponders are expensive (as well as the necessary equipment and technology associated with the transponders and calculation of data received from transponders), they are used infrequently on segments of the deployment cable, often at intervals of 100 meters or more. In operation, cable is laid down not in perfect lines or patterns on the seabed, and thus the exact location of entire sections of cable (and the relevant nodes) are effectively unknown between the transponders. Recording seismic signals from the seabed requires proper positioning of the node and/or sensor and different orientations and improper configurations inhibit the coupling of the seismic sensor to the seabed, providing poor or inaccurate data. Still further, the deployment and retrieval of transponders requires additional equipment on the deployment vessel and additional time for the handling of such transponders.
A need exists for an improved method and system for the monitoring and/or guiding of cable deployed with nodes on the seabed, and in particular one that eliminates all or substantially all of the transponders typically used in such applications and eliminates the use of a USBL system. A new system is needed that is more cost effective, allows better positioning and accuracy of deployed nodes, and allows for the real-time (or near real-time) guidance of the deployed nodes.